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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 28 March 2013 by dana1981

Yesterday The Economist published an article about climate sensitivity – how much the planet's surface will warm in response to the increased greenhouse effect from a doubling of atmospheric CO2, including amplifying and dampening feedbacks. For the most part the article was well-researched, with the exception of a few errors, like calling financier Nic Lewis "an independent climate scientist." The main shortcomings in the article lie in its interpretation of the research that it presented.

For example, the article focused heavily on the slowed global surface warming over the past decade, and a few studies which, based on that slowed surface warming, have concluded that climate sensitivity is relatively low. However, as we have discussed on Skeptical Science, those estimates do not include the accelerated warming of the deeper oceans over the past decade, and they appear to be overly sensitive to short-term natural variability. The Economist article touched only briefly on the accelerated deep ocean warming, and oddly seemed to dismiss this data as "obscure."

The Economist article also referenced the circular Tung and Zhou (2013) paper we addressed here, and suggested that if equilibrium climate sensitivity is 2°C to a doubling of CO2, we might be better off adapting to rather than trying to mitigate climate change. Unfortunately, as we discussed here, even a 2°C sensitivity would set us on a path for very dangerous climate change unless we take serious steps to reduce our greenhouse gas emissions.

Ultimately it was rather strange to see such a complex technical subject as climate sensitivity tackled in a business-related publication. While The Economist made a good effort at the topic, their lack of expertise showed.

Some commenters skeptical of the severity of projected climate change have recently seized on two sources to argue that the climate may be less sensitive than many scientists say and the impacts of climate change therefore less serious: A yet-to-be-published study from Norwegian researchers, and remarks by James Annan, a climate scientist with the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).

While the points skeptics are making significantly overstate their case, a look at recent developments in estimates of climate sensitivity may help provide a better estimate of future warming. These estimates are critical, as climate sensitivity will be one of the main factors determining how much warming the world experiences during the 21st century.

Climate sensitivity is an important and often poorly understood concept. Put simply, it is usually defined as the amount of global surface warming that will occur when atmospheric CO2 concentrations double. These estimates have proven remarkably stable over time, generally falling in the range of 1.5 to 4.5 degrees C per doubling of CO2.* Using its established terminology, IPCC in its Fourth Assessment Report slightly narrowed this range, arguing that climate sensitivity was “likely” between 2 C to 4.5 C, and that it was “very likely” more than 1.5 C.

The wide range of estimates of climate sensitivity is attributable to uncertainties about the magnitude of climate feedbacks (e.g., water vapor, clouds, and albedo). Those estimates also reflect uncertainties involving changes in temperature and forcing in the distant past. But based on the radiative properties, there is broad agreement that, all things being equal, a doubling of CO2 will yield a temperature increase of a bit more than 1 C if feedbacks are ignored. However, it is known from estimates of past climate changes and from atmospheric physics-based models that Earth’s climate is more sensitive than that. A prime example: Small perturbations in orbital forcings resulting in vast ice ages could not have occurred without strong feedbacks.

Water Vapor: Major GHG and Major Feedback

Water vapor is responsible for the major feedback, increasing sensitivity from 1 C to somewhere between 2 and 4.5 C. Water vapor is itself a powerful greenhouse gas, and the amount of water vapor in the atmosphere is in part determined by the temperature of the air. As the world warms, the absolute amount of water vapor in the atmosphere will increase and therefore so too will the greenhouse effect.

That increased atmospheric water vapor will also affect cloud cover, though impacts of changes in cloud cover on climate sensitivity are much more uncertain. What is clear is that a warming world will also be a world with less ice and snow cover. With less ice and snow reflecting the Sun’s rays, melting will decrease Earth’s albedo, with a predictable impact: more warming.

There are several different ways to estimate climate sensitivity:

Examining Earth’s temperature response during the last millennium, glacial periods in the past, or periods even further back in geological time, such as the Paleocene Eocene Thermal Maximum;

Looking at recent temperature measurements and data from satellites;

Examining the response of Earth’s climate to major volcanic eruptions; and

Using global climate models to test the response of a doubling of CO2 concentrations.

These methods produce generally comparable results, as shown in the figure below.

The grey area shows IPCC’s estimated sensitivity ranges of 2 C to 4.5 C. Different approaches tend to obtain slightly different mean estimates. Those based on instrumental temperature records (e.g., thermometer measurements over the past 150 years or so) have a mean sensitivity of around 2.5 C, while climate models average closer to 3.5 C.

The ‘Sting’ of the Long Tail of Sensitivity

Much of the recent discussion of climate sensitivity in online forums and in peer-reviewed literature focuses on two areas: cutting off the so-called “long tail” of low probability\high climate sensitivities (e.g., above 6 C or so), and reconciling the recent slowdown in observed surface warming with predictions from global climate models.

Being able to rule out low-probability/high-sensitivity outcomes is important for a number of reasons. For one, the non-linear relationship between warming and economic harm means that the most extreme damages would occur in very high-sensitivity cases (as Harvard economist Marty Weitzman puts it, “the sting is in the long tail” of climate sensitivity). Being able to better rule out low probability/high climate sensitivities can change assessments of the potential economic damages resulting from climate change. Much of the recent work arguing against very high-sensitivity estimates has been done by James Annan and Jules Hargreaves.

The relatively slow rate of warming over the past decade has lowered some estimates of climate sensitivity based on surface temperature records. While temperatures have remained within the envelope of estimates from climate models, they have at times approached the 5 percent to 95 percent confidence intervals, as shown in the figure below.

However, reasonably comprehensive global temperature records exist only since around 1850, and sensitivity estimates derived from surface temperature records can be overly sensitive to decadal variability. To illustrate that latter point, in the Norwegian study referred to earlier, an estimate of sensitivity using temperature data up to the year 2000 resulted in a relatively high sensitivity of 3.9 C per doubling. Adding in just a single decade of data, from 2000 to 2010, significantly reduces the estimate of sensitivity to 1.9 C.

There’s an important lesson there: The fact that the results are so sensitive to relatively short periods of time should provide a cautionary tale against taking single numbers at face value. If the current decade turns out to be hotter than the first decade of this century, some sensitivity estimates based on surface temperature records may end up being much higher.

So what about climate sensitivity? We are left going back to the IPCC synthesis, that it is “likely” between 2 C and 4.5 C per doubling of CO2 concentrations, and “very likely” more than 1.5 C. While different researchers have different best estimates (James Annan, for example, says his best estimate is 2.5 C), uncertainties still mean that estimates cannot be narrowed down to a far narrower and more precise range.

Ultimately, from the perspective of policy makers and the general public, the impacts of climate change and the required mitigation and adaptation efforts are largely the same in a world of 2 or 4 C per doubling of CO2 concentrations where carbon dioxide emissions are rising quickly.

Just how warm the world will be in 2100 depends more on how much carbon is emitted into the atmosphere, and what might be done about it, than on what the precise climate sensitivity ends up being. A world with a relatively low climate sensitivity — say in the range of 2 C — but with high emissions and with atmospheric concentrations three to four times those of pre-industrial levels is still probably a far different planet than the one we humans have become accustomed to. And it’s likely not one we would find nearly so hospitable.

Comments

"Ultimately it was rather strange to see such a complex technical subject as climate sensitivity tackled in a business-related publication. While The Economist made a good effort at the topic, their lack of expertise showed."

The Economist has published articles on science and technology for at least 40 years, indeed it usually has an entire section devoted to that topic. For some years it has also taken the warmist line. That they are now raising questions for the AGW side marks something of a turning point in my view.

"Just how warm the world will be in 2100 depends more on how much carbon is emitted into the atmosphere, and what might be done about it, than on what the precise climate sensitivity ends up being." That would only be true if the precise climate sensitivity was a postive number materially different from zero. At zero the volume of CO2 in the atmosphere would make no difference to temperature.

Climate Sensitivity - is it rather a strange subject for in-depth analysis by The Economist? Not really.

Surely economists would do well to understand it. And even if it were not, in its Science & Technology section, The Economist does cover subjects that cannot be relevant to economics. For instance, last week it looked at the evolution of animals 500m years ago. Perhaps then, with all the economic gloom, an occasional diversion is good.

The Economist did a poor job printing the article discussed by this post. They did also run a front-piece editorial which struck a better note, concluding "If the world has a bit more breathing space to deal with global warming, that will be good. But breathing space helps only if you actually do something with it." Still, I don't think it makes up for the dreadful article.

I just thought it was strange for The Economist to try and tackle this subject because they clearly lack the expertise to properly interpret the research. It seemed like they sent an intern to find a bunch of papers to reference, dumped them into an article, and didn't really know what to say about them.

elsa @2 - given that climate sensitivity is indisputably different from zero, I guess you're agreeing that Zeke's statement is true.

A zero climate sensitivity is so far beyond reality as to border on insanity. That's like saying gravity might stop at any moment -- that the effect of gravity could turn out to be near zero. A zero climate sensitivity is that insane.

Best estimates for climate sensitivity today are still between 2.5 and 4 C per doubling. Even the lowest in that range, 2.5 C, will turn out to be very, very bad for civilization, and at this point in time it seems vanishingly improbable that we will not shoot past and beyond a mere doubling.

People don't seem to recognize how much of a change that minimum reasonable estimate represents. And heaven help us if it's even just a little higher (like in that most likely case of 3 C or even 3.5 C).

Although deniers are trying to spin it otherwise (part of the message they're trying to sell that their movement is growing, and any material that they might spin as being skeptical is evidence of that), this isn't the first time The Economist has commented on climate science, or with a arguably skeptical angle. In 2011, for example,

Before you take the SUV out for a celebratory spin, though, it is worth bearing in mind that this is only one study, and, like all such, it has its flaws. The computer model used is of only middling sophistication, Dr Schmittner admits. That may be one reason for the narrow range of his team's results

That is the statement from the previous article you reference. Not exactly an anti AGW statement. It looks like this other article was just a piece on this one paper.

I was disappointed overall with the tone of the Economist piece, because they are generally "sound" on the science of climate change. It shows that propaganda about a temperature "hiatus" and lower carbon sensitivity has gained some traction in the more rational media.

It has already cited by contrarians around the web as though it was somehow supportive of their position.

But look on the bright side - David Rose and the Daily Mail it wasn't. Not even close.

So what does all this amount to? The scientists are cautious about interpreting their findings. As Dr Knutti puts it, “the bottom line is that there are several lines of evidence, where the observed trends are pushing down, whereas the models are pushing up, so my personal view is that the overall assessment hasn’t changed much.”

But given the hiatus in warming and all the new evidence, a small reduction in estimates of climate sensitivity would seem to be justified: a downwards nudge on various best estimates from 3°C to 2.5°C, perhaps; a lower ceiling (around 4.5°C), certainly. If climate scientists were credit-rating agencies, climate sensitivity would be on negative watch. But it would not yet be downgraded.

Best estimate is still 3 C...but maybe it will be concluded to be 17% less.

Both articles have their caveats. Both also emphasize a skeptical angle with regards to best estimate climate sensitivity (the 2011 article describes it as "good news". Failing to do so would be boring, and the media doesn't like boring.

Clearly, this isn't the first time The Economist has had a skeptical angle in their articles, although they certainly have had better-quality articles on climate-related issues.

tobyjoyce,

It has already cited by contrarians around the web as though it was somehow supportive of their position.

What deniers look for is movement towards their position from any realm (media, scientists, politicians, real or perceived), which is ultimately a strong desire not to legislate reductions in greenhouse gas emissions. Even the perceived slightest movement warrants heavy spin, as they believe it helps with public perception, as in "hey look, the media is starting to figure out we're right".

My observations are that media on the whole has moved away from science in general, due in part to cost-cutting measures and the big economic downturn a few years ago. What remains isn't as good of quality. At the same time, denial is also on the decline over the last few years (public polls indicate that as well). ClimateGate is virtually dead, and I'm not seeing as many contrarian views covered in media, other than the usual suspects. Could be the whole ClimateGate thing killed contrarian credibility, and media felt duped in the end. Deniers have to stretch these days to find something supportive of their movement.

For the most part, The Economist has doing a surprisingly good job covering anthropogenic climate change. However, this was a very dissapointing piece by The Economist.

As Dana noted climate sensitivity is complex (as is also evidenced by The Economist's long missive), and the inability of the article's author to correctly interpret the myriad of papers and data sources showed. That they erroneously consider Nic Lewis to be a climate scientist was a huge red flag that the article's author was not informed enough to tell the real science from the chaff. All in all, The Economist piece is an exercise in wishful thinking.

Climate sensitivity is the fake skeptics' final fall-back position for inaction, so I can understand why they will cheer anything that they perceive to support their agenda. But even if climate sensitivity to doubling is "only" near 2 C, that does not disappear anthropogenic warming, nor does it disappear ocean acidification, nor does it disappear rising sea levels, loss of Arctic sea ice, receding glaciers or increases in certain extreme weather phenomena, nor does it change the fact that on our current emissions path we will likely quadruple CO2 levels before 2100.

Regardless of what equilibrium climate sensitivity is, we should be reducing GHG as quickly as possible because we have most likely already missed the boat to keeping warming to 2 C or less.

Odd then, that some people seem to be trying to console themselves that everything will be OK or that aggressive action to reduce GHG emissions is not required by trying to argue for lower climate sensitivity. That approach is very much backwards and amounts to nothing more than wishful thinking.

The climate sensitivity seems to me what divides the warmists from the deniers. (-snip-). Take on board the full implications of the statement "To illustrate that latter point, in the Norwegian study referred to earlier, an estimate of sensitivity using temperature data up to the year 2000 resulted in a relatively high sensitivity of 3.9 C per doubling. Adding in just a single decade of data, from 2000 to 2010, significantly reduces the estimate of sensitivity to 1.9 C." and you ought to see what I mean. To claim that we know with 95% confidence (which actually the IPCC has never done) that the average temperature will rise by x degrees cannot sit comfortably alongside this statement.

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Moderator Response: [DB] Off-topic sloganeering snipped. Perhaps in the 2 years and 7 months that have elapsed you should have taken the time to actually learn something about the climate models (one of the basis for determining CS) you so thoroughly are uninformed about.

I read the Economists' article and all I can say is dana1981 seems to be correct. For example, the article gives pretty much equal weight between IPCC's 3C sensitivity estimate and Berntsen's 1.9C sensitivity. It backs up Berntsen's estimate with other studies, and mentions even lower estimates several times. The casual reader could easily come away with a 'feeling' the sensitivity is likely 2C or lower. What the Economist omits is the many studies that point to a 4C sensitivity or higher. This presentation of a false balance should be familiar to watchers of Fox News.

From the Economist article "[if climate sensitivity is low] more adaptation rather than more mitigation might be the right policy" It might be if mitigation costs are high, but what if they are low? Why does a magazine that calls itself 'the Economist' omit the HALF of that equation they would seem most qualified to estimate? By omission, the article maintains the myth that mitigation costs are high, and hence the 'adaptation' decision is solely dependent on those durned climate scientists and their tea leaves.

From the Economist article: "If climate scientists were credit-rating agencies, climate sensitivity would be on negative watch" But actually, if climate scientists were credit-rating agencies, then they would rate Wallstreet mortgage derivatives 'AAA' and sell them to pensioners. The Economist missed an $11 trillion housing bubble, yet the credit-rating agencies that helped create it are still the 'gold standard' when it comes to judging value? Such is the world we live in, apparently.

The uncertainty about climate change is now essentially ALL on the side of evaluating economic impacts versus costs of mitigation. Given its recent failings, I guess I can't blame the Economist for focusing on climate uncertainty instead. But its no service to their readers. The cost of mitigation is variously estimated to be 1% of GDP, which is pretty trivial. And the benefits? What is the benefit to keeping Manhattan and its credit-rating agencies from drowning? Whatever that is, the Economist won't be talking about it.

elsa@12: Please read the rebuttal to the bogus argument that "CO2 is just a trace gas". The difference between a CO2 concentration of 0.0% and the pre-industrial concentration of 0.3% is the difference between a frozen Earth and a nice comfortable Earth that can support a population of billions. Small amounts of lots of things make a big difference. It's not the percentage that matters, it's the doubling of it that matters.

It is one of the few times I have read someone so wildly inaccurate in their assessment that they could not even aspire to being wrong. I do not think you read the same Economist article as everyone else. You are talking about an article whose second sentence is "But the problem is not going to go away.", and which finds the climate sensitivity debate "Hardly reassuring". Perhaps you should have read a bit farther than you did.

ubrew2, the Economist is usually sound on the science, but is slavish in its promotion of fracking and natural gas as a "bridge fuel". Here, we can see the same trends as the housing bubble emerging. But fracking and bridge fuels are at least more "respectable" ideas than fake scepticism. Obama's new Energy Secretary seems to also hold to those views.

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Moderator Response:

[JH] I deleted elsa's post because it was nothing more than sloganeering.

"To claim that we know with 95% confidence (which actually the IPCC has never done) that the average temperature will rise by x degrees cannot sit comfortably alongside this statement."

I think everyone here would agree. So where does this 'claim' come from? Pulled off a less than honest/accurate blogsite?

There's no need to emphasise uncertainy where everyone here is aware of it. If we knew to a preciser degree what conditions would be like 30, 60 and 100 years from now, policy prescriptions would be easier to formulate. The perennial counter-argument of the 'skeptical' - that we have no certainty on the matter - somehow never seems to make it felt on them that this is cause for greater concern than better knowledge. Surely uncertainy means we should say, "it could be better or worse than our middle estimates," and therefore consider the full spectrum of risk. But when 'skeptics' say, "it's uncertain," they mean for us to hear, "it's going to be much less of a problem than we need to worry about." That's the rational disjoint in their whole thesis. Even skeptic Roger Pielke Senior advances this quite resonable proposition.

"I don't think we know the consequences of what we're doing. But our footprint on the environment is more than just CO2: It's nitrogen deposition, it's the other black carbon, the aerosols, it's land-use change. And so we put all of these things together and say, "How can we come up with a policy that reduces our impact on the environment?" Because we don't know the consequences....

The problem is, we don't know if we're pushing ourselves toward or away from some negative impact. That's the problem. We could be making ourselves actually less likely to have some drought pattern, but since we don't know, to me the prudent pattern is to try to minimize our impact."

Knowing less than Pielke and less than many here, I reflect that human industry has added 40% more CO2 to the atmosphere in a couple of centuries - an astonishing amount - and the surface temperature of the earth has definitely increased in that time. More uncertainty about causes and the future makes me feel more, not less concerned. The 'skeptics' have a much more rosy view of the future, but no good reason for it without being highly selective in their reading. The 'skeptical' blogs are their filter.

shoymore @15.As Dana1981 says, the Economist article is not factually incorrect yet its analysis of those facts is in error. My own take is that the error is so bad, the article is entirely misguided.

You will note that the context of concluding phrase "Hardly reassuring" that you quote (with to much assurance in my mind). The quote is not saying climate is a problem. It is actually saying climatology isn't up to the job.Such is the content of this article from the top 15-year flat temperatures to bottom. The author is continually exhibiting the symptoms of underlying denial.

As for elsa, consider her/his flip from insisting ECS=0 to suggesting ECS assessments flip about far too wildly 'and you ought to see what I mean' when I tell you that elsa's position no longer fits comfortably alongside itself.

My interpretation of the article in The Economist is that it is not nearly so skeptical or luke-warmist as you think. In particular, a fair reading of the last paragraph shows that the author is indeed expressing a concern about the actual state of the climate at 4 degrees C, not just the question of how precisely it will be known.

In other words, the question it addressing is not the academic issue of scientific precision but rather the implications for people living in the world. It is, after all, a newspaper.

I agree with nealjking that the articles in the Economist (and yes shoyemore I have read both of them all the way through) are not particularly skeptical. But as various others have noted above there is a hint of what most on here would call denialism. I doubt that the Economist could change from true warmist to denier in one leap but there is definitely an underlying change of heart in there.

Quite rightly there is also a questioning of climatology, as MA Roger has pointed out. As can be seen from Ed Hawkins' two graphs the current temperature is now outside the 75% confidence limit and touching on the 95% limit.

(-snip-). Scaddenp asks me by what physical process I arrive at a climate sensitivity that is close to zero. (-snip-).

elsa @20 - the only problem with the Economist article is that it's behind the times. They barely touch on the critical accelerated warming of the deep oceans, and they don't recognize the flaws in the recent low sensitivity studies (which is not surprising, since they're not climate experts). It's not "denialism", it's just incomplete research leading to flawed interpretations and conclusions.

I will agree that 'denialism' is all about ignoring evidence. The difference is that The Econimist isn't just ignoring inconvenient data, they just haven't yet caught up with the current state of climate research.

The argument that CO2 is too small to matter is possibly the most lame of all positions to take, and demonstrates an ignorance of the science that (IMO) disqualifies a person from participating in any rational, educated discussion.

I don't think I said that MA Roger agreed with doubting climatology, I think he said that one of the things the Economist did was doubt climatology, which it did.

With regard to the models you can see from this article that there is a very wide spread of forecasts, which makes them less helpful than they would otherwise be, and the point has now been reached where the actual temperature is hitting the bottom 95% confidence limit. What we are not told is the age of the models under investigation.

With regard to the physical process you have not addressed the point. Yes people have looked at actual temperature and CO2 concentrations but that does not supply an equation linking the two derived from physics. All it is doing (and the models likewise) is fitting the data to the theory. Of course such a theory will appear to be true.

The argument whether CO2 is too small to matter lies at the heart of the debate between us. I suppose at least you do say IMO!

False. The article is close to 3,000 words, all of it citing climate science reports and studies as its foundation. How can you possibly claim that that means the article doubts climatology?

What we are not told is the age of the models under investigation.

False (and off-topic). There is a wealth of information available on all of the models, and they are continually being improved and expanded. That no one has sent a personal delivery boy to hand you a gilded copy of the details of the models is no one's fault. You need to go do some research and educate yourself.

All it is doing (and the models likewise) is fitting the data to the theory.

False. This is off-topic, so I will not bother to discuss it here, but your misunderstanding of the science is pitiful. Both the models and how CO2 affects temperatures are entirely derived from physics. You are quoting nonsense that you've read at idiocracies like WUWT.

You are repeatedly, embarassingly wrong. Follow the links above, open your mind, and actually read something. Repeating falsehoods that you read somewhere on the Internet does not make them true.

Considering the number of models and the large range of estimated sensitivities, it seems that the estimates are little more than guesses.

Then there is the work of Ferenc M. Miskokze who has performed analysis that claims that any effect that adding CO2 to the atmosphere would have on climate is compensated for by negative feedback of H20 in the upper atmosphere.

H2O is modeled as a positive feedback to added CO2 in the lower atmosphere because CO2 warming allows more H20 into the atmosphere which causes even more warming. But in the upper atmosphere the situation is reversed. More CO2 causes the upper atmosphere where LWIR radiates to space to cool. If the earth radiates less in the upper atmosphere then the result will be a net gain in energy by the earth. Cooler upper atmospheric temperatures in the upper atmosphere reduce H2O content. Less H2O contallowsowes more heat to travel up to the upper atmosphere causing temperatures there to rise and the global energy output to space to rise. This negative feedback in the upper atmosphestabilizesizes the climate to changes in green house gasses.

There are certainly equations and calculations relating the radiative forcings and the concentrations of the greenhouse gases. They are a bit complex, because the impacts of the different gases depend on the distributions of the gases throughout the atmosphere, and even at a given frequency they do not add linearly. Nonetheless, there is nothing mysterious about them.

Your concern about the scantiness of CO2 is ill-placed, for two reasons:

- The bulk of the atmospheric molecules do not absorb in the infrared (IR). Therefore, it does not matter how many there are of them. It is only the greenhouse gases (primarily H2O and CO2) that interact with the IR, so the question boils down to, Are there enough H2O and CO2 molecules to catch the IR photons? It turns out that there are. What matters is the absolute numbers of H2O and CO2 molecules; their proportion to other molecules in the atmosphere is totally irrelevant.

- Concerning the proportion of CO2 relative to H2O: Here one could make an argument that H2O swamps CO2, for the IR spectral bands they both absorb. However, H2O does not get above 10 km in altitude, whereas CO2 gets up to 100 km. In the theory of the greenhouse effect, the high-altitude molecules have a much more significant impact on the steady-state radiative balance than the low-altitude molecules, so CO2 dominates H2O for their common IR bands.

I spent some time with Miskolczi to try to clarify his argument. I had to stop it after awhile due to the press of other matters, but in principle the discussion is still open, so I don't want to say too much. However, it is fair to say that his approach is rather non-standard.

A discussion group of supporters of his formed to try to rewrite his argument in a more transparent way, but after several months they became discouraged and dissolved.

Question: inasmuch as the ocean, especially the deep ocean, has recently been accumulating heat at a faster than "average" rate due to ENSO variables, do climate sensitivity estimates include factors such as theoretical tipping points, e.g. abrupt methane releases from clathrates? If not, why not?

Rob Painting reminds me of an analogy I proposed some time ago to explain the irrelevance of the smallness of the concentration of greenhouse gases (GHGs).

Think of a city square where a politician is to give a talk. Many people are there to listen; but a handful have been hired by his opponents, to assassinate him. How will the likelihood of their success depend on their relative proportions?

Basically, the more assassins there are, the greater the likelihood that one of them gets him: one or two assassins could easily be stopped by the Secret Service, but if there are a lot of them, it's much more likely someone will get a shot.

How will this be affected by the number of non-assassins? Basically, not at all: non-assassins are not there to protect the politician, they are just there to listen. They will not be looking for trouble. They are irrelevant.

Therefore, to first order, the likelihood of success by the assassins will be the same if they represent 100% of the attendees as if they represent 1/100-% of the attendees. (In fact, in a slightly more realistic case, the more non-assassins there are, the more distracted the Secret Service will be; so the chances of assassination increase as the proportion of assassins drops.)

Now think of the politician as being the infrared photon, the assassins as being the greenhouse gas molecules, and the non-assassins as being the non-greenhouse-gas molecules. In the same way, the proportion of GHG molecules is to 1st order irrelevant to the GHG capability; and, due to pressure broadening, if the number of non-GHG molecules is increased, the GHG capability even increases.

scaddenp Since you ask me "to get down to tin tacks" I will. I see no reason to believe that a doubling of the concentration of CO2 (from one very low figure to another slightly higher but still very low one) will result in a temperature change of very much. You mention of 3.7W/m2 without explaining where this number is derived from. Perhaps you can tell us why this particular one is right?

With regard to the physical process you have not addressed the point. Yes people have looked at actual temperature and CO2 concentrations but that does not supply an equation linking the two derived from physics. All it is doing (and the models likewise) is fitting the data to the theory.

The models are sets of equations. They are not fit to observations of temperature. They are tuned to emulate certain processes, but not trends. The physics of infrared absoprption of greenhouse gases is derived from empirical measurements, which are the spine of climate models. The impatience you are reading above probably comes from the fact that this information is basic to an understanding of models, and your views are very under-informed.

The following paper is one of the seminal calculations of the atmospheric greenpouse effect, from 1978.

Thank you for your comment. Stripped of its niceties you seem to say that the models are not derived from physics, but from measurements. But if that is the case they are not testing a theory at all. They are fitting (or tuning as you put it) the data to the theory so of course they will appear to work.

Since you ask me "to get down to tin tacks" I will. I see no reason to believe that a doubling of the concentration of CO2 (from one very low figure to another slightly higher but still very low one) will result in a temperature change of very much.

No reason to believe? But where does your doubt take you? Do you investigate your own assumption?

Human industry has added 100 parts per million of CO2 to the body of the atmosphere. Would you happily swallow 100 ppm of your body weight in cyanide, believing that such a small amount will have negligible effects? I would suggest more study before you attempt to demonstrate why tiny amounts of something must perforce be of little consequence.

There is a massive dent in the ozone layer the size of Australia over the South Pole from a few hundred parts per billion increase of ozone depleting gases in the atmosphere. The process is different (catalytic), but the point is the same.

Rather than scale, investigate the power of CO2 as a greenhouse gas. It is a tiny fraction of the atmospheric composition, 0.039%, but GHGs, which, increase the surface temperature by 33C, comprise only a small part of the atmospheric total of gases. The effect of CO2 in terms of greenhouse forcing is relative to its proportion in the total of greenhouse gases and its particular power power as a greenhouse gas (different gases have different absorptive power). Its contribution is calculated to be somewhere between 9 and 20% of the greenhouse effect.

If wikipedia is not enough for you, you can refer to the science papers I linked above (Ramanathan & Coakley (1978), the ones linked at the wiki page, (eg, this one) and have a rummage in google scholar.

Don't wait for your lack of belief to inform you. Apply any curioisity you may have.

I just wrote this comment to the Economist article. Do fellow SKS'sers think it's a good point?

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Perhaps the writer of this article ought to have done a bit of thinking before writing this misleading article.

They portray the lower estimates of climate sensitivity, from a few researchers (around 1.9-2.0°C per CO2 doubling), as being somehow "safe" because this makes some hope that we could probably handle that amount of warming.

Like many people trying to make the sceptical/denialist case, they do not think it through. If the policy makers of the world thought such a value of sensitivity was "safe", what do you think would happen? There would be few, if any, attempts to reduce emissions, so atmospheric levels of CO2 would carry on rising faster.

Here's a question for the sceptics - what makes you think levels would stop just at a doubled value from pre-industrial times (280 -> 560ppm)? Why would humanity not, in due course, double that again to 1120 ppm which would lead, even using the rose tinted "get out of jail free" sensitivity of 2°C, to 4°C of warming, which no one sane can dispute would be highly dangerous.

To those who don't think we could get to 1120ppm with just our fossil fuel, fracked gas, tars sands etc remember that the Arctic is warming a lot more than most and there is a lot of tundra/permafrost up there already starting to melt, which will be releasing large amounts of CO2 as the frozen organic material decomposes. Not to mention the increased out-gassing of CO2 from the oceans as they warm too.

So, even if these lower sensitivity figures are valid, we are still facing a very dangerous situation and articles such as this one, that try to make out we are not, are reckless and irresponsible.

Stripped of its niceties you seem to say that the models are not derived from physics, but from measurements. But if that is the case they are not testing a theory at all. They are fitting (or tuning as you put it) the data to the theory so of course they will appear to work.

The measurements referred to are of how CO2 (and other gases) absorb radiation in the atmosphere. The physics of climate models are based on these measurements. You said there was no physical link to CO2 and temperature in climate models:

Yes people have looked at actual temperature and CO2 concentrations but that does not supply an equation linking the two derived from physics.

The information provided you is the 'link' you think is missing - and that is only a fraction of the work done on the very matter you assume is 'missing'. Read the papers I linked above and see for yourself.

Of course GCMs are based on empirical data. If you think that scientific theories must operate without using actual data, then you have an unbelievably misbegotten notion of science.

Your assertion was that models are fit to temperature data. You are entirely, utterly and completely wrong about that. They are based on the physics of climate processes. They are not tuned to temperatures.

No insult intended - you are very ill-informed on these matters. Check out the links, learn more about it, and try here again after that.

Your request for a "single" equation linking CO2 and global temperature is sort of like asking for the single equation that shows that a Boeing 787 can fly, or the single equation that relates total US tax revenues to personal income.

...but for a starting point, if you want to question that CO2 absorbs IR radiation, perhaps you can ask the people at Licor that sell off-the-shelf IR gas analyzers that use IR to measure atmospheric CO2 concentrations for the equations they use. This is stuff that is old hat.

...the models are not derived from physics, but from measurements. ... They are fitting (or tuning as you put it) the data to the theory so of course they will appear to work.

False.

Casual readers are asked to please recognize the completley vacuous and programmed nature of Elsa's comments. She seems to merely recite the most childish of climate denial myths, while making no effort to use the power of the Internet to correct her ignorance.

This is the stuff that denial is made of; smoke, pixie dust, ignorance and frustration.

Nick @39 - yes, actually The Economist filmed an interview with the two contributors to this article. One of the points they made was that lower sensitivity might matter if our climate policy was suitable for a world with 3°C sensitivity; however, in reality our policy is woefully inadequate. Thus from a policy perspective, 3°C or 2.5°C or even 2°C sensitivity makes little to no difference. In any case we're not doing nearly enough to mitigate climate change.

As I've said, this article is not written from a denialist perspective. They recognize the problem and that we need to solve it; they've just been convinced by evidence for slightly lower climate sensitivity that really is quite weak.

Since one of her few remaining posts on my blog has been quoted in order to demonstrate the historical antecedents of Elsa's inveterate denial, I'll just point out that before my blog she wrote some very strange stuff in the Guardian's CiF, the content of which along with posts on my blog were so irrational that I had to show her the door. Of course, I was accused of censorship etc (corresponding to the victimhood status inherent in all climate change denial) but I mention this now because, as then, her posts are extremely disruptive - which I believe is one purpose of denialist activism, as well as to infer doubt where, in fact, none exists. Is it wise to take such posts seriously?

By the way, if SkS considers this post to be off-topic or in any way inappropriate, please remove it.

On the subject of sensitivity: recently, I've considered that, in the context of such discussions, we may be rather missing the point.

Climate sensitivity has, for some time now, been one of the few scientific uncertainties on which sceptics can call to support their position. Their argument has been - roughly - that if sensitivity was low, feedbacks did not materialise or have a significant effect, then climate change would be considerably less of a problem (therefore requiring considerably less spending on the solutions).

What has been under-emphasised in my opinion is the relationship between cause and effect. Predictions of temperature rises for a doubling of CO2 represent a way of calculating how much additional energy may be retained by the ecosystem - oceans, land, air - but climate sensitivity does not tell us how the ecosystem will react, by how much, or when.

I mention this because, from a real-world perspective, the issue is not by how much the temperatures will rise, but what effect any rise will have - and here we are in much less certain territory. Consider the melting ice and the way that weather appears to be showing increases in localised extremes. Both these effects have, until recently, been discussed as effects we might see mid-century, as heat/energy accumulates. These timescales now seem hopelessly inadequate, suffering from the conservatism I believe has materially biased much climate science. (Hardly surprising when you're under attack all the time, of course).

We will, as a matter of retrospective measurement, determine the true value of energy retension for a doubling of CO2. Meanwhile, I hope others might agree that we need to emphasise not so much the theoretical aspect of climate sensitivity, but the dawning realisation that massive - and catastrophic - disruption of the climate may be wrought by as little as a 2 degree increase in temperatures, the same figure touted for so long as a 'safe' limit.

I would also like to add one other point: the homogenization of regional temperature records that give us the 0.8 degree anomaly to date, or the putative figures for future doubling of CO2, also conceal a more disturbing metric. Temperatures do not rise evenly across the globe, and therefore the effects of even the warming to date cannot be understood merely by averaging the change. Local anomalies will be the cause of local effects - the Arctic is a particularly compelling example given the effect its local temperature anomalies of up to 2.28 degrees C (GISS) are already having on N. Hemisphere climate patterns - and I think the actual effects of significant local temperature increases on our lives are being disguised by using the global metric in our discussions.

The first-order warming from the enhanced greenhouse effect (EGHE) is due to radiative forcing, and this is derived on the basis of radiative transfer theory. The general idea goes like this:

- The temperature profile of the atmosphere makes it generally cooler as you go higher.

- A beam of radiation that starts at ground level and proceeds upwards to the sky will be roughly at ground temperature at the bottom; its temperature will be reduced along the beam as you proceed upwards, remaining roughly the same as the surrounding greenhouse gas (GHG) at each level. (The local beam temperature is reflected in the spectral density of radiation at that point.)

- When the GHG has become so thin that the remainder of the GHG gas (up to the very top of the atmosphere) has total optical depth of 1 (optical depth = line integral of the coefficient of absorption), the local beam temperature stops tracking the local gas temperature profile, and the beam "escapes".

- So the bottom line is that the radiation of the beam at ground level is characteristic of the ground-level temperature; the radiation of the beam as it escapes the atmosphere is characteristic of the temperature at the altitude at which the GHG optical depth = 1 (as measured from that altitude out into space).

- Because the high-altitude temperature is lower than the ground-level temperature, LESS radiation escapes than started out at the bottom of the beam. That is the mechanism of the greenhouse effect (GHG). In radiative equilibrium, the radiation escaping has to equal the solar radiant energy absorbed by the Earth, so this defines a characteristic temperature at that critical point.

- So now, when extra GHG is added to the atmosphere, it permeates the atmosphere and raises the GHG concentration everywhere; in particular, CO2 gets as high up as 100 km. The result is that the optical depth at every altitude INCREASES, thus the point at which optical depth = 1 moves UPWARD; therefore, the local temperature at this new critical point is cooler, implying that the intensity of escaping radiation is REDUCED. This means that the radiant power lost is NO LONGER in radiative equilibrium with the solar radiation.

- So what happens? The Earth is now absorbing more radiant energy than it is losing, so it warms up. The temperature increase also propagates up through the atmosphere over time, such that the temperature increases at every altitude. When the temperature at the NEW critical point (where optical depth = 1) increases enough to match the original temperature at the OLD critical point (before the extra GHG was added), the escaping radiation will again match the incoming solar radiation. The temperature everywhere will be a bit higher. This is the mechanism of the enhanced greenhouse effect (EGHE).

- This walk-through of the radiative transfer physics of radiative forcing is taken from the textbook Principles of Planetary Climate, by Raymond Pierrehumbert (U. Chicago). There are some additional complexities due to the fact that the the infrared radiation has to be considered as made up of separate spectral bands appropriate to each type of GHG molecule, but this is the basic idea.

- The total magnitude of the warming is determined also by 2nd-order feedback effects that entail the complexity of the cllmate system. But the calculation briefly described above sets the "first step".

Elsa's education seems to be lamentably lacking in the basic science that demonstrates that CO2 can absorb infrared radiation.

If the scientific literature itself is too challenging, perhaps she might consider a visual proof such Iain Stewart's demonstration, or Pieter Trans' very similar demonstration. These are very simple yet very graphic corroborations of the fact that CO2 is not transparent to infrared.

The basis for the 3.7 W/M2 per doubling is this paper, cited in the IPCC's AR4. Myhre et al 1998. However, if you want to dig deeper than that you need to start delving into Radiative Transfer science. The Eqn of Radiative Transfer; Line-By-Line, Narrow Band & Broad Band radiative transfer codes; Databases of spectral data such as HiTran and Geisa; the history of the developement of this through the 1950's and 1960's; The particular role the Nimbus 3 satellite played in 1969 in confirming previous observations and so on. Lots of research.